Lens implementation in JavaScript

lens.js

const curry = fn => (...args) =>
  args.length >= fn.length ? fn(...args) : curry(fn.bind(undefined, ...args))

const always = a => b => a

const compose = (...fns) => args => fns.reduceRight((x, f) => f(x), args)

const getFunctor = x =>
  Object.freeze({
    value: x,
    map: f => getFunctor(x),
  })

const setFunctor = x =>
  Object.freeze({
    value: x,
    map: f => setFunctor(f(x)),
  })

const prop = curry((k, obj) => (obj ? obj[k] : undefined))

const assoc = curry((k, v, obj) => ({ ...obj, [k]: v }))

const lens = curry((getter, setter) => F => target =>
  F(getter(target)).map(focus => setter(focus, target))
)

const lensProp = k => lens(prop(k), assoc(k))

const lensPath = path => compose(...path.map(lensProp))

const view = curry((lens, obj) => lens(getFunctor)(obj).value)

const over = curry((lens, f, obj) => lens(y => setFunctor(f(y)))(obj).value)

const set = curry((lens, val, obj) => over(lens, always(val), obj))